1. Field of the Invention
The present invention relates to a print data generation apparatus and a print data generation method for quantizing image data, and employing the quantized image data to generate print data based on density patterns.
2. Description of the Related Art
For image data quantization, there is one well known method that employs a density patterns or an index pattern that describes an arrangement of binary print data for respective areas in a single pixel of image data. According to this method, 256-valued image data are quantized to obtain N-valued data, which are smaller than the 256-valued data, and density patterns of N levels that correspond to values 0 to N−1 of the N-valued data, are prepared and employed according to the value of 0 to N−1 so as to determine printing (dot) being on or off for each of unit pixels (unit areas), which form a single pixel. That is, the density pattern defines an arrangement of a number of dots that corresponds to each of N levels. Since this method requires only a small amount of processing of quantization, quantization at high resolution can be performed especially in short period of time.
Among the methods for performing quantization using density patterns, a method for employing, for one level, a plurality of density patterns, for which different dot arrangement patterns are provided is known (Japanese Patent Laid-Open No. H09-046522 (1997)). In the case of employing the same (one) density pattern for one level, periodicity of the pattern is increased, and accordingly, interference by the pattern tends to occur. In contrast to this, the method described in Japanese Patent Laid-Open No. H09-046522 (1997) can eliminate the periodicity of a pattern to avoid the interference due to the pattern, by employing the plurality of patterns sequentially for one level.
The smaller the size of dots formed with ejected ink is made, the more granularity due to the formed dots can be reduced. In this extent, increasing the printing resolution is preferable for a reduction in the granularity. In this case, the method employing the density patterns makes the resolution of the density pattern higher than the resolution of image data to be quantized. For example, in a case where quantization by an error diffusion is performed for image data having a resolution of 600 dpi×600 dpi to obtain binary print data having a resolution of 1200 dpi×1200 dpi, a density pattern is a pattern in which 2 areas (unit pixels)×2 areas (unit pixels) correspond to one pixel of image data.
With the configuration described in Japanese Patent Laid-Open No. H09-046522 (1997), in which a plurality of density patterns are employed for each level of quantized data, a dot pattern for level 1, i.e., for a case where one dot is on (printed), can be a pattern in which a dot is on at one of the four areas of 2 areas×2 areas. In a case for level 2, i.e., a case where two dots are on, six patterns are available for an arrangement in which dots are on at two of the four areas. This is the matter of a calculation for a combination that is performed by selecting two areas out of four areas, and is represented by 4C2. Similarly, in a case for level 3, this calculation is represented by 4C3. In a case of processing performed for a higher resolution, e.g., a case where quantization is performed for image data having a resolution of 600 dpi×600 dpi to obtain binary print data having a resolution of 2400 dpi×2400 dpi, a density pattern employed is a pattern of 4 areas×4 areas. Then the number of density patterns employed for one level is also increased, and for example, in the case for level 1, i.e., a case where one dot is on at only one area, 16 patterns are available. As described above, when the printing resolution is increased, the number (or the types) of density patterns that can be set for each level is greatly increased.
However, when the number of density patterns available for one level is increased, a granularity reduction effect that has been provided by an increase in resolution may be canceled. More specifically, there are areas in the density patterns where a dot (print data of “1”) is not arranged in accordance with the levels, and these areas are recognized as whitishness in a printed image, i.e., as low-density image portions. Then, when a plurality of density patterns having different dot arrangements are sequentially employed, the whitishness may appear non-periodically. In such a case, the non-periodic whitishness is eccentrically distributed, and noticeable granularity may be observed in the printed image. Further, as described above, since the number of density patterns employed for one level is increased as the printing resolution becomes higher, the probability that whitishness will be unevenly distributed is increased.